1. Field of the Invention
The present invention relates to spark plugs for use in internal combustion engines of motor vehicles, cogeneration systems and gas-pressure feed pumps or the like and, more particularly, to a spark plug having long operating life and a related manufacturing method.
2. Description of the Related Art
In the related art, attempts have heretofore been made to provide spark plugs as igniting means for internal combustion engines of motor vehicles or the like.
Each of these spark plugs generally includes a center electrode and a ground electrode between which a spark discharge gap is provided. Applying a high voltage across the center electrode and the ground electrode allows a spark discharge to take place in the spark discharge gap, thereby igniting an air-fuel mixture.
With the development of internal combustion engines with increased performances in maintenance-free statuses, the spark plugs have been required to have long operating life. To satisfy such requirements, an attempt has heretofore been made to form a spark plug with a center electrode having a spark discharge portion provided with an Ir alloy tip in face of the spark discharge gap.
Here, the Ir alloy tip and a center electrode base material body, made of Ni-based alloy, have a big difference in thermal expansion coefficients. Therefore, the Ir alloy tip is liable to drop off from the center electrode base material body when exposed to thermal stresses in frequent times. To address such an issue, it has been a common practice to employ laser welding to bond the Ir alloy tip to the center electrode base material body via a fused layer having a thermal expansion coefficient in a substantially intermediate level between those of the Ir alloy tip and the center electrode base material body. This allows a reduction in thermal stress acting on the center electrode, thereby permitting the Ir alloy tip and the center electrode base material body to ensure increased bonding capability.
In such a laser welding method, the Ir alloy tip and the center electrode base material body are preliminarily unitized to each other by resistance welding or the like, after which a laser beam is irradiated onto an entire circumferential periphery of the Ir alloy tip while turning the same about an axis thereof.
Here, the center electrode has laser-welding capability that remarkably depends on contours of the Ir alloy tip and the center electrode base material body exposed to a position at which the laser beam is irradiated. If the contours of the Ir alloy tip and the center electrode base material body exposed to the laser beam irradiating position are irregular, a joint portion between the Ir alloy tip and the center electrode base material body is fused in an uneven fusing pattern with the resultant difficulty of having adequate bonding capability. To address such an issue, an Ir alloy tip processed in a columnar configuration has been used to provide a fixed profile at all times during the rotation of the Ir alloy tip when performing welding step.
However, a large number of fabricating steps need to be performed for processing the Ir alloy tip in a precisely columnar shape. With a view to addressing such a problem, U.S. Pat. No. 6,885,137 discloses a spark plug that is manufactured in a process wherein even if an Ir alloy tip has a non-roundness shape in cross section on a plane perpendicular to an axis of the Ir alloy tip, a roundness tolerance is specified such that the Ir alloy tip is bonded to a center electrode base material body with bonding capability nearly equal to that obtained with a columnar shaped Ir alloy tip.
In addition, the above U.S. patent also discloses a rod-like Ir alloy tip, formed in a polygonal shape more than hexagonal shape in cross section, which is more preferable to be used as an Ir alloy tip for the purpose of satisfying the requirements mentioned above.
Meanwhile, from a standpoint of an increase in operating life of the spark plug depending on a wearing speed of the Ir alloy tip, it is advisable for the Ir alloy tip to have a square shape in cross section. That is, the Ir alloy tip formed in such a square shape is particularly effective for a spark plug of the sidewise-facing ground electrode type with a ground electrode placed in face of an outer circumferential periphery of a center electrode.
As shown in FIGS. 29 to 31, in welding a square shaped rod-like Ir alloy tip 92 to a center electrode base material body 91, the square shaped rod-like Ir alloy tip 92 is welded to the center electrode base material body 91 with an outline circle of the center electrode base material body 91 placed in an area slightly outside a circumscribed circle of a square shape of the Ir alloy tip 92. During such welding process, uneven differences occur in distance between a circumferential sidewall of the center electrode base material body 91 and sidewalls of the square shaped rod-like Ir alloy tip 92. Thus, a fused layer 95 (see FIG. 32) tends to have unevenness in thermal expansion coefficient after laser welding has been completed.
With a structure of a center electrode 90 as shown, for instance, in FIG. 31, a sidewall 921 of the square shaped rod-like Ir alloy tip 92 is spaced from a circumferential sidewall 911 of the center electrode base material body 91 by a distance L1 in a direction vertical to the sidewall 921 of the square shaped rod-like Ir alloy tip 92.
Further, a corner 922 of the square shaped rod-like Ir alloy tip 92 is spaced from the circumferential sidewall 911 of the center electrode base material body 91 by a distance L2 in a direction along a diagonal line 923 of the square shaped rod-like Ir alloy tip 92.
Thus, the distance L1 between the sidewall 921 of the square shaped rod-like Ir alloy tip 92 and the circumferential sidewall 911 of the center electrode base material body 91 becomes greater than the distance L2 between the corner 922 of the square shaped rod-like Ir alloy tip 92 and the circumferential sidewall 911 of the center electrode base material body 91.
Accordingly, the fused layer 95 (see FIG. 32) resulting from laser welding has a Ni-rich area, influenced with material components (Ni or the like) of the center electrode base material body 91, in the vicinity of a vertical region 912 between the sidewall 921 of the square shaped rod-like Ir alloy tip 92 and the circumferential sidewall 911 of the center electrode base material body 91. Thus, the fused layer 95 has a thermal expansion coefficient deviated to that of the center electrode base material body 91 in the vicinity of the vertical region 912.
On the contrary, the fused layer 95 has an Ir rich area, influenced with material components (Ir or the like) of the square shaped rod-like Ir alloy tip 92, in the vicinity of a diagonal region 913 of the square shaped rod-like Ir alloy tip 92. Thus, the fused layer 95 has a thermal expansion coefficient deviated to that of the square shaped rod-like Ir alloy tip 92 in the vicinity of the diagonal region 913 thereof.
Thus, the fused layer 95 becomes hard to have an overall circumference having a thermal expansion coefficient in the vicinity of a value intermediate between those of the square shaped rod-like Ir alloy tip 92, containing Ir, and the center electrode base material body 91 containing Ni or the like.
Further, even if laser welding is performed under a condition to minimize a difference in thermal expansion coefficients in various areas of the fused portion on an entire circumferential periphery thereof, the fused layer 95 becomes rich in Ni or the like as a whole, causing a difficulty to occur in forming the fused layer 95 so as to have the thermal expansion coefficient in the vicinity of the intermediate value between those of the square shaped rod-like Ir alloy tip 92 and the center electrode base material body 91.
Furthermore, the corner 922 and a center of the square shaped rod-like Ir alloy tip 92 have a greater distance along the diagonal line than a distance between the sidewall 921 of the square shaped rod-like Ir alloy tip 92 and the center thereof. Therefore, the innermost fused region 95a of the fused layer 95 is hard to reach the center of the square shaped rod-like Ir alloy tip 92 with a concurrence of the unfused region 96 being left as shown in FIG. 32. As a result, the spark plug of the related art suffers from increased thermal stresses when exposed to thermal shocks in repeated cycles during operations in the internal combustion engine.
As set forth above, the spark plug formed in such a structure mentioned above has incapability of achieving a reduction in thermal stress with the resultant difficulty of ensuring adequate bonding capability.
Accordingly, under circumstances where the spark plug is exposed to thermal shocks in rapid heating and rapid cooling in repeated cycles, there is a fear of flaking or cracking 97 occurring in the fused layer 95 at the joint portion of the center electrode 90.
Moreover, U.S. Pat. No. 6,724,132 discloses a spark plug of a sidewise-facing ground electrode type having a center electrode provided with a noble metal tip formed in a square shape in cross section. With the spark plug of the center electrode formed in such a structure, the square shaped noble metal tip has an outer diameter larger than that of a center electrode base material body. That is, an inscribed circle of the square shaped noble metal tip is larger in diameter than the outer diameter of the center electrode base material body.
Therefore, the noble metal tip can be welded to the center electrode base material body only upon completely assembling the center electrode base material body to a porcelain insulator. Thus, an issue arises with the occurrence of deterioration in productivity of the spark plug. In addition, the fused layer of the center electrode needs to be placed in a position protruding from an end face of the porcelain insulator. This causes a limitation to occur in the positional relationship between a welding position and the end face of the porcelain insulator.